A paper sheet processing apparatus such as a postal item processor that processes postal items such as postcards or sealed letters includes, e.g., a pick up device, a discrimination device (OCR), an accumulation device, a reject (RJ) accumulation device, a switchback device, a convey path connecting the respective devices, a gate that sorts carried paper sheets (postal items) to the respective devices, and others. A plurality of paper sheets loaded in a supply section of the pick up device are picked up one by one by the pick up device, separated into each paper sheet and are then supplied to the discrimination device. The discrimination device discriminates the paper sheets, determines a destination of the paper sheets, e.g., the RJ accumulation device or the accumulation device, and also determines whether each paper sheet is to be passed through a transfer route, e.g., the switchback device and then turned back. Afterward, each paper sheet is transferred to the determined device through the convey path and a gate mechanism, and the carried sheet is subjected to various processes in this device.
As the pick up device in the paper sheet processing apparatus, an adsorption type pick up device that adsorbs each paper sheet by a negative pressure and picks up the paper sheet has been suggested (e.g., JP-A 2008-280139 [KOKAI] and JP-A 2001-335165 [KOKAI]). This pick up device includes an air adsorption structure that uses a perforated belt and an air chamber to adsorb each paper sheet and a separation roller that adsorbs and separates a second paper sheet to prevent two paper sheets from being simultaneously picked up. Further, the pick up device includes an optical sensor that detects presence/absence or a density of paper sheets in the supply unit from which the paper sheets are supplied. When the sensor determines the absence of paper sheets by transmission, the supply unit is operated to feed the next paper sheet to a pick up position at a given speed.
In the above-described paper sheet pick up device, paper sheets are often densely stacked in e.g., a portion where paper sheets having a relatively small thickness are continuous in a paper sheet bundle placed in the supply unit. Therefore, the supply unit pushes the paper sheets at a speed higher than an ideal speed, and paper sheet jam is apt to occur near the pick up position. In this case, the earliest paper sheet cannot be stably picked up, or skew or overlapping feed of the paper sheets may possibly occur. On the contrary, a portion where paper sheets having a relatively large thickness are continuous, feed of the paper sheets by the supply unit is delayed, and hence pick up of the paper sheets may become intermittent. In such cases, a processing speed of the paper sheet processing apparatus deteriorates.